Systems for creating a 3-D representation of a given portion of space have a variety of potential applications in many different fields. Examples are automotive sensor technology (e.g. vehicle occupant detection and classification), robotic sensor technology (e.g. object identification) or safety engineering (e.g. plant monitoring) to name only a few. As opposed to conventional 2-D imaging, a 3-D imaging system requires depth information about the target scene. In other words, the distances between one or more observed objects and an optical receiver of the system need to be determined. A well-known approach for distance measurement, which is used e.g. in radar applications, consists in timing the interval between emission and echo-return of a measurement signal. This so called time-of-flight (TOF) approach is based on the principle that, for a signal with known propagation speed in a given medium, the distance to be measured is given by the product of the propagation speed and the time the signal spends to travel back and forth.
In case of optical imaging systems, the measurement signal consists of light waves. For the purposes of the present description, the term “light” is to be understood as including visible, infrared (IR) and ultraviolet (UV) light.
Distance measurement by means of light waves generally requires varying the intensity of the emitted light in time. The TOF method can e.g. be using the phase-shift technique or the pulse technique. In the phase-shift technique, the amplitude of the emitted light is periodically modulated (e.g. by sinusoidal modulation) and the phase of the modulation at emission is compared to the phase of the modulation at reception. In the pulse technique, light is emitted in discrete pulses without the requirement of periodicity. In phase-shift measurements, the modulation period is normally in the order of twice the difference between the maximum measurement distance and the minimum measurement distance divided by the velocity of light. In this approach, the propagation time interval is determined as phase difference by means of a phase comparison between the emitted and the received light signal. Such phase comparison requires synchronization of the demodulation signal with the emitted light signal. Due to the high propagation speed given by the velocity of light, a fundamental difficulty encountered in distance measurements based on the pulse technique or the phase-shift technique resides in the required temporal resolution of the measurement device. In fact, a spatial resolution in the order of centimeters requires a temporal resolution in the order of 10−11 seconds (10 ps).
With both the pulsed light and phase shift methods, time intervals are generally measured in the electrical domain. Therefore, electric propagation times and delays which affect the synchronization in the measurement device have a determining influence on the measurement accuracy. Current problems in this respect are unknown variations and drifts of the electric propagation times on the signal lines and of the delays in the electronic components. For instance, there may be variations between devices of the same type occur, for example because of tolerances in the production processes (e.g. semiconductor production). Additionally, drifts may occur during operation, e.g. due to temperature variations or component ageing. These variations and drifts have a detrimental influence on measurement accuracy.
As a result, efforts have been made to overcome this problem by providing a more reliable synchronization. It has been proposed, for example by Schwarte in WO98/10255, to provide an optical feedback path from the light emitting module to one or more sensor cells of the light receiving camera. As shown in DE 44 39 298 by Schwarte, a phase reference for synchronization purposes can be obtained by guiding the emitted light without reflection to the receiver e.g. through an optical fibre.
An improved 3-D imaging system has been proposed in the international patent application PCT/EP2006/060374. This patent application discloses a 3-D imaging system comprising an illumination unit for emitting light into a scene and an imaging sensor for imaging the scene by detecting scattered light. The system also comprises an evaluation unit, for determining distance information related to the scene on the basis of light propagation time, and synchronization means for providing synchronization information to the evaluation unit. To overcome the above-mentioned drawbacks, the synchronization means comprises means for generating an electrical reference signal in the illumination unit, the reference signal being directly derived from the emitted light.